A fuel cell stack is configured to include a plurality of stacked fuel cells and a fastening mechanism for electrically connecting therebetween and maintaining air tightness therein (e.g., a seal). In particular, when an applied fastening force is too strong, damage may be caused to an inside of a fuel cell and when the applied fastening force is too weak, it may be difficult to obtain required air tightness and electrical connection. Therefore, there is a need to apply an appropriate fastening force.
FIG. 1 illustrates the existing stack fastening structure in which an end plate 2a is disposed within a fuel cell stack 1a to apply the fastening force to a fuel cell stack 1a and a fixed fastening band 3a is fastened with the end plate 2a by a bolt 4a. As illustrated in FIG. 1, the existing stack fastening structure is not adjustable in length based on a dimension deviation of components in the fuel cell, such as a gas diffusion layer or a gasket, and when the fastening force is reduced due to the long-term use, it may be difficult to compensate for the fastening force unless the fuel cell stack 1a is disassembled and then an insertion body is inserted thereinto or the fastening band 3a manufactured at different dimensions is replaced.
Instead of the fastening band 3a, a bolt that penetrates through the fuel cell stack 1a and a combination of a nut coupled with the bolt may adjust the fastening force applied to the fuel cell stack. However, when a pitch of a thread formed in the used bolt 4a is too large, it may be difficult to finely adjust the fastening force and to quantitatively determine the fastening force when the fuel cell stack 1 is fastened.
FIG. 2 illustrates an elastic body 3b inserted between the fastening mechanism 1b and the stack 2b to measure the fastening force according to the related art. As illustrated in FIG. 2, the elastic body 3b is inserted between the fastening mechanism 1b and the stack 2b to measure a compressed length of the elastic body 3b, thereby measuring the fastening force. However, considering that the applied fastening force per one fastening band is generally 500 to 600 kgf, a change in the length of the elastic body 3b based on a change in the fastening force is minimal, and as a result, it may be difficult to measure the change in the length of the elastic body 3b. 
For example, when the elastic body having a modulus of elasticity of 10 kgf/mm is used, since an original length of the elastic body 3b is equal to or more 50 mm, a length of the fastening mechanism 1b is too large, and as a result, it is difficult to be mounted. On the other hand, the elastic body 3b having a modulus of elasticity of 500 kgf/mm has a changed length as much as 0.02 mm when the modulus of elasticity is changed as much as 10 kgf, and as a result, it is difficult to be mounted.